Sleep disturbance, often cyclic in nature, are reported by most totally blind people. These sleep disturbances are associated with abnormalities in the circadian rhythms of core body temperature, cortisol and melatonin secretion, and urinary excretion. There have been seven case reports demonstrating a loss of synchronization of circadian rhythms to the 24-hour day in blind subjects, although in the majority cases the masking effects of activity, food intake, posture, and sleep have not been controlled during measurement of circadian output. Preliminary data conducted under minimal masking conditions show severe circadian disruption in a majority of cases in blind persons, characterized by sleep disturbances and/or abnormal phase position of the core body temperature amplitude with advancing age. In addition, blind subjects with disturbed sleep had significantly lower temperature lower temperature amplitude than those without such disturbances. On the basis of these results, four testable hypotheses are proposed: (1) that the sleep complaints found in the majority of blind persons are associated with an abnormal synchronization of the endogenous circadian rhythms of core body temperature and plasma melatonin with the geophysical day: (2) That the endogenous circadian rhythms of body temperature and circulating plasma melatonin concentrations are synchronized with a normal phase relationship to the geophysical day in that minority of blind person without sleep complaints: (3) That the amplitude of the endogenous circadian rhythms of core body temperature is lower in blind persons with sleep complaints than in those without sleep complaints: and (4) That the amplitude of the endogenous circadian rhythms of core body temperature and circulating plasma melatonin levels are reduced with advancing age in blind person. We will conduct experiments to assess the endogenous amplitude and phase core temperature and plasma melatonin cycles in young and older totally blind people, correlate these findings with subjective and objective measures of sleep disruption and sleep quality, and compare the results with similar data in a population of young and older sighted person. This work has significant implications for understanding the pathophysiology of circadian rhythms disorders in the blind and the effects of aging on the output of the circadian pacemaker in the absence of light perception.